The invention relates to a process for producing a holographic film and to a holographic film obtainable by the process.
Holographic films can be produced, for example, with the aid of special photopolymer formulations. Thus, for example, WO 2008/125199 A1 describes a photopolymer formulation which contains polyurethane-based matrix polymers, an acrylate-based writing monomer and photoinitiators. If a layer of the photopolymer formulation is cured, the writing monomer and the photoinitiators are embedded with an isotropic distribution in space in the resulting polyurethane matrix. In this way, a film into which holograms can be incorporated by exposure to light is obtained.
This can be effected by means of the superposition of two coherent light sources, a three-dimensional structure which in general can be described by a regional change in the refractive index (refractive index modulation Δn) forming in the medium. Such structures are referred to as holograms, which can also be described as diffractive optical elements. The optical functions which are performed by such a hologram depend on the specific exposure to light.
For the uses of photopolymer formulations, the refractive index modulation Δn produced by the holographic exposure to light in the photopolymer plays the decisive role. During the holographic exposure to light, the interference field of signal and reference light beam (in the simplest case, that of two plane waves is formed by the local photopolymerization of, for example, highly refractive acrylates at sites of high intensity in the interference field in a refractive index grating. The refractive index grating in the photopolymer (the hologram) contains all information of the signal light beam. By exposing the hologram only to the reference light beam, the signal can then be reconstructed. The strength of the signal reconstructed in this manner in relation to the strength of the incident reference light is referred to as diffraction efficiency or DE below. In the simplest case of a hologram which forms from the superposition of two plane waves, the DE is obtained from the quotient of the intensity of the light diffracted in the reconstruction and the sum of the intensities of incident reference light and diffracted light. The higher the DE, the more efficient is a hologram with respect to the quantity of reference light which is required in order to make the signal visible with a fixed brightness.
Highly refractive acrylates are capable of producing refractive index gratings having a high amplitude between regions with low refractive index and regions with high refractive index and hence permitting holograms with high DE and high Δn in photopolymer formulations. It should be noted that DE is dependent on the product of Δn and the photopolymer layer thickness d. The greater the product, the greater the possible DE (for reflection holograms). The width of the angular range in which the hologram becomes visible (reconstructed), for example in the case of exposure to monochromatic light, depends only on the layer thickness d. In the case of exposure of the hologram to, for example, white light, the width of the spectral region which can contribute to the reconstruction of the hologram likewise depends only on the layer thickness d. The smaller d, the greater are the respective acceptance widths.
If it is intended to produce bright and readily visible holograms, a high Δn and small thickness d should be strived for, in particular so that DE is as large as possible. This means that the higher Δn, the more latitude achieved for establishing the layer thickness d for bright holograms without loss of DE. The optimization of Δn in the optimization of photopolymer formulations is therefore of outstanding importance (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).
Attempts have therefore been made to date to realize as high a Δn as possible by changing the composition of the photopolymer formulations used for the production of the holographic films. However, it has been found that the photopolymer formulations developed in laboratory experiments cannot be used without considerable problems in some cases for the industrial production of holographic films.
Such an industrial production process is described, for example, in European Patent Application not laid open as yet and having the application number 09001952.2. In this process, a photopolymer formulation is applied to a substrate material and then dried at elevated temperature. In this way, holographic media in the form of films can be obtained.
If the photopolymer formulations optimized in laboratory experiments for a high Δn are used in the process described above, on the one hand films which do not have sufficient mechanical stability are obtained in many cases, so that crushing of the applied photopolymer formulation occurs during winding onto a roll. This is particularly disadvantageous since the films cannot be wound up as rolls without destruction.
On the other hand, it is true that it is possible to use films which have sufficient mechanical stability. However, holograms which were written into these films do not have the desired high Δn values.
It has therefore not been directly possible to date to produce holographic films on the industrial scale which both have the necessary mechanical stability and strength and are suitable for writing of holograms having high Δn values.